FIGURE 5.

Ion permeability of AmGABA receptors. (a) Reversal potential analysis of current–voltage curves obtained by applying −50 mV/+50 mV voltage ramps in the presence of 100‐μM GABA. Reversal potentials of AmRDL‐containing GABA receptors are sensitive to increased Cl⁻ concentrations but insensitive to external Na⁺ concentration changes. The opposite results are obtained with AmGRD/LCCH3, for which the reversal potential is only sensitive to increasing sodium concentrations. (b) Semi‐log representation of reversal potential values. Mean values of reversal potentials obtained are plotted versus the logarithm of the Na⁺ or Cl⁻ concentrations. This allows the calculation of the slope of the regression line for each current studied. The absolute value of the slope for a monovalent permeant ion should be close to 58 mV according to the Nernst equation. For AmRDL receptors, the calculated slopes for Na⁺ and Cl⁻ are −47 and −3 mV, respectively, meaning that these receptors are mainly permeable to Cl⁻. On the contrary, these values are +2 and +50 mV respectively for AmGRD/LCCH3, suggesting that these receptors are mainly permeable to Na⁺, exhibiting a substantial permeability to K⁺. (c) AmRDL and AmGRD/LCCH3 currents recorded in the presence or in the absence of external Ca²⁺. (d) Concentration‐dependant curves obtained in Xenopus oocytes expressing AmRDL or AmGRD/LCCH3 with increasing concentrations of the anionic open channel blocker PCCP⁻ (n = 4). PCCP⁻ blocked only the 100‐μM GABA‐evoked currents elicited by AmRDL activation and therefore is a pharmacological tool useful in discriminating between the anion or cation permeability of insect GABA receptors